Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per weight than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Each electrode is in intimate contact with a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a layer at the solid liquid interface region. This is accomplished by absorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The absorbed charge is held in this region by opposite charges in the solid electrode to generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
Nonaqueous ultracapacitors use an organic salt solution as an electrolyte. Low levels of moisture and loss of electrolyte both contribute to degradation of the ultracapacitor cells. Such degradation adversely affects both performance and life of an ultracapacitor. Hence, proper sealing of an ultracapacitor cell is paramount to the manufacture of a high performance, long-lived cell. Proper sealing has been difficult because of the aggressive chemical nature of many of the aprotic polar solvents used as cell electrolyte solvents. Many of the common adhesives such as epoxies, cyanate esters, silicones and ethylene vinyl acetates fail mechanically, lose adhesion or permit defusion of solvent through sealant. The common failure of sealants requires that a secondary container be used to completely contain and seal the ultracapacitor cell. The present invention provides a hermetic and leak proof seal through a primary seal with the current collectors of the ultracapacitor thus eliminating the need for a secondary container. Further, the present invention uses a resealable closure mechanism that allows repair of the cell and removal of internal moisture by release of gas pressure.
The invention relates to an ultracapacitor that comprises at least one cell comprising two solid, nonporous current collectors, two porous electrodes separating the current collectors, a porous separator between the electrodes and an electrolyte occupying pores in the electrodes and separator. The cell is sealed with a reclosable hermetic closure.
The invention also relates to a stack of ultracapacitor cells comprising a plurality of bipolar double layer ultracapacitor cells in stacked relationship, at least one cell comprising porous, oppositely charged electrodes with an ionically charged separator disposed between the electrodes. The stack includes a non-porous current collector between each cell with each current collector having adjoining polarized electrodes of different cells bonded thereto. An electrolyte saturates the electrodes and separators. At least one cell of the stack is sealed with a reclosable hermetic closure.
In another aspect, the invention relates to a method of making an ultracapacitor, comprising providing a multilayer cell comprising two solid, nonporous current collectors, two porous electrodes separating the current collectors, a porous separator between the electrodes and an electrolyte occupying pores in the electrodes and separator. The cell is then sealed with a reclosable hermetic closure.
In a final aspect, the invention relates a method of making a stack of ultracapacitor cells. In the method, a plurality of bipolar double layer ultracapacitor cells are provided in stacked relationship. At least one cell comprises porous, oppositely charged electrodes with an ionically charged separator disposed between the electrodes. a non-porous current collector is provided between each cell with each current collector having adjoining polarized electrodes of different cells bonded thereto. The electrodes and separators are saturated with electrolyte and at least one cell of the stack is sealed with a reclosable hermetic closure.